This invention relates to a solid-state imaging device using a four-phase charge-coupled device (CCD) having four gate electrodes for a single photodiode and to a method of manufacturing the solid-state imaging device.
A CCD is a semiconductor device that has a structure where plural phase charge transfer electrodes are formed on a semiconductor channel region formed in a semiconductor substrate via a thin gate insulating film of a thickness ranging from 0.1 to 0.2 xcexcm, for example, and that transfers a charge signal by applying pulse voltages to the plural phase charge transfer electrodes, respectively, and changing the potential of the channel region under the electrodes.
FIG. 11 shows the structure of one cell of a conventional four-phase CCD. FIG. 11A is a top view of the cell. FIG. 11B is a sectional view taken along line XIBxe2x80x94XIB in FIG. 11A. FIG. 11C is a sectional view taken along line XICxe2x80x94XIC of FIG. 11A.
As shown in FIG. 11A, a CCD solid-state imaging device is composed of a photodiode 11 constituting a light-receiving section (pixel section) and four phase charge transfer electrodes formed in the direction of, for example, column of the photodiode 11. On the photodiode 11, the charge transfer electrodes are not formed. Between photodiodes 11 and, for example, in a region in the direction of row, charge transfer electrodes are formed via a gate insulating film 2 above a semiconductor substrate 1, thereby forming a charge transfer region as shown in FIG. 11B, where its sectional view is shown. The charge transfer electrodes formed in a region in the direction of, for example, column between photodiodes 11 constitute an inter-connection layer region for supplying voltage to the individual charge transfer electrodes in the charge transfer region as shown in FIG. 11C, where its sectional view is shown. Although not shown, on the regions except for the light-receiving section, a shading film is formed of a metal layer film made of, for example, Al.
The charge transfer electrodes shown in the figure are formed of three layers of polysilicon film: a first charge transfer electrode is made of a first-layer polysilicon film; a second and fourth-layer charge transfer electrodes are made of a second-layer polysilicon film, and a third charge transfer electrodes is made of a third-layer polysilicon film. The individual charge transfer electrodes are isolated from each other by an oxide film. The oxide film is formed by patterning the polysilicon film of each layer using, for example, lithography and etching techniques and by thereafter oxidizing the patterned polysilicon layer.
To form an oxide film for isolating the individual charge transfer electrodes from each other as described above, it is necessary to pattern each polysilicon film separately. This approach therefore requires mating margins for patterning in processing each layer, which makes it difficult to subminiaturize cells. Furthermore, to secure the mating margins, the area of a photodiode 11 is reduced, leading to the problem of lowering the sensitivity.
As shown in FIG. 11C, in the interconnection layer region, three layers of polysilicon films are stacked one on top of another, so the step height between the interconnection layer region and the photodiode region 11 is large. In forming a shading film, this prevents the shading film from being formed sufficiently over the step portion and therefore light may enter the regions other than the photodiode 11, causing an erroneous signal.
In contrast, there is a method of thickening a shading film so that the shading film may be formed sufficiently even at the step portion. With this method, however, when the shading film on the photodiode region 11 is etched away, it is difficult to etch the film away sufficiently. Because the shading film is formed thicker in the periphery of the photodiode region 11, the amount of light reaching the photodiode 11 is smaller, lowering the sensitivity of the solid-state imaging device. For this reason, it is desirable that the thickness of the shading film should not be made thicker.
Moreover, since it is necessary to form and process three layers of polysilicon film, the processes are long and complex.
As described above, with the conventional solid-state imaging device and method of manufacturing the device, since the charge transfer electrodes are composed of three layers of polysilicon films, the step height between a region where three layers of polysilicon films are stacked one on top of another, such as an interconnection layer region, and the photodiode region is large, which prevents a shading film from covering the step portion sufficiently, resulting in the problem of permitting light to enter the regions other than the photodiode, thus causing an erroneous signal.
Furthermore, because the three layers of polysilicon films must be patterned separately, it is necessary to secure mating margins for patterning, leading to the problem of making it difficult to subminiaturize cells.
In addition, because it is necessary to process the three layers of polysilicon films, this results in the problem of making the manufacturing processes long and complex.
The object of the present invention is to provide a high-density solid-state imaging device which can suppress the generation of erroneous signals by reducing the step height between the interconnection region and the charge transfer electrode region and photodiode region and whose manufacturing processes are simple, and a method of manufacturing the solid-state imaging device.
The foregoing object is accomplished by providing a solid-state imaging device comprising: a plurality of light-receiving elements formed on a semiconductor substrate; and a set of four charge transfer electrodes that are formed via a gate insulating film in the regions between the light-receiving elements and that are applied with four different pulse signals, the set of four charge transfer electrodes being arranged repeatedly, wherein a first charge transfer electrode, a fourth charge transfer electrode, and part of a second charge transfer electrode in the set of charge transfer electrodes are made of a first conductive film, a third charge transfer electrode and the remaining portion of the second charge transfer electrode in the set of charge transfer electrodes are made of second conductive film, the first conductive film is joined to the second conductive film in the second charge transfer electrode, an oxide film formed by thermally oxidizing the first conductive film isolates the first charge transfer electrode from the second charge transfer electrode, the second charge transfer electrode from the third charge transfer electrode, and the third charge transfer electrode from the fourth charge transfer electrode, and the end of the second conductive film is formed so as to locate on the oxide film on the first conductive film.
The solid-state imaging device may further comprise a first conductive material portion formed of the second conductive film joined to the sidewall of the first charge transfer electrode in a set of charge transfer electrodes adjacent to the fourth charge transfer electrode; and a second conductive material portion formed of the second conductive film joined to the sidewall of the fourth charge transfer electrode in a set of charge transfer electrodes adjacent to the first charge transfer electrode.
In the solid-state imaging device, the first and second conductive films may be made of a polysilicon film.
The foregoing object is also accomplished by providing a method of manufacturing solid-state imaging devices, comprising: the step of forming a gate insulating film on a semiconductor substrate; the step of forming a first conductive film on the gate insulating film; the step of removing the first conductive film in part of a second charge transfer electrode region and in a third charge transfer electrode region so that the first conductive film may have a strip pattern; the step of thermally oxidizing the surface of the remaining first conductive film; the step of removing, on the first charge transfer electrode region side, part of a thermal oxidation film formed in the thermal oxidation process on the first conductive film constituting part of the second charge transfer electrode; the step of removing the thermal oxidation film in the region between the fourth charge transfer electrode and the first charge transfer electrode in a set of charge transfer electrodes adjacent to the fourth charge transfer electrode and the first conductive film so that they may have a strip pattern; the step of forming a second conductive film; the step of forming a first to fourth strip charge transfer electrodes by removing the second conductive film using as a mask a first resist film having an opening on part of the insulating film parallel with and partially overlapping with the first strip conductive film and remaining on the second charge transfer electrode and on the region between the fourth charge transfer electrode and the first charge transfer electrode in a set of charge transfer electrodes adjacent to the fourth charge transfer electrode; and the step of removing the second conductive film, the thermal oxidation film, and the first conductive film using as a mask a second resist film having an opening on a light-receiving element region on the first to fourth strip charge transfer electrodes.
In the method of manufacturing solid-state imaging devices, the step of forming a first to fourth strip charge transfer electrodes by removing the second conductive film may include the step of etching the second conductive film using anisotropic etching techniques.
As described above, with the solid-state imaging device of the present invention, because the four charge transfer electrodes to which four different pulse signals are applied are made of the first and second layer conductive films, the step heights due to the charge transfer electrodes are reduced as compared with a conventional equivalent where the charge transfer electrodes were made of three layers of conductive films. This makes it possible to form a shading film at a high covering rate, which helps prevent light from entering the regions other than the light-receiving region, thus avoiding the generation of erroneous signals.
Because the thermal oxidation film of the first conductive film, which is generally excellent in insulation, isolates the first charge transfer electrode from the second charge transfer electrode, the second charge transfer electrode from the third charge transfer electrode, and the third charge transfer electrode from the fourth charge transfer electrode, it is possible to isolate the charge transfer electrodes from each other reliably.
Because the distance between the charge transfer electrodes on the channel region through which charges are transferred is determined by the thickness of the thermal oxidation film, the distance between the charge transfer electrodes can be made constant. This makes it easy to set the impurity concentration distribution in the channel region.
Furthermore, because the distance between the charge transfer electrodes can be made smaller than the minimum dimension the lithography process will allow, the transfer efficiency of charges can be improved.
Since the end of the second conductive film is formed so as to locate on the oxide film on the first conductive film, there is no need of securing mating margins in the lithography process, which helps reduce the area of cells and form a highly integrated solid-state imaging device. When the cell area is not reduced, the area of the light-receiving element can be increased, improving the sensitivity of the solid-state imaging device.
With the solid-state imaging device of the present invention where the first conductive material portion is formed of the second conductive film joined to the sidewall of the first charge transfer electrode in a set of charge transfer electrodes adjacent to the fourth charge transfer electrode and the second conductive material portion is formed of the second conductive film joined to the sidewall of the fourth charge transfer electrode in a set of charge transfer electrodes adjacent to the first charge transfer electrode, because the first conductive material portion is joined to the fourth charge transfer electrode and the second conductive material portion is joined to the first charge transfer electrode, these conductive material portions function as the fourth or first charge transfer electrode. Namely, this produces the same effect of the fourth or first charge transfer electrode being extended. This enables the conductive material portions to shorten the distance between adjacent sets of charge transfer electrodes, thereby making the distance smaller than the minimum dimension the lithography process will allow. This helps improve the transfer efficiency of charges.
With the solid-state imaging device of the present invention where the first and second conductive films are formed of polysilicon films, an insulating film excellent in insulation can be formed by thermally oxidizing the polysilicon film constituting the first conductive film.
With the method of manufacturing solid-state imaging devices according to the present invention, the first to fourth charge transfer electrodes are formed of the first and second conductive films formed on the semiconductor substrate via the gate insulating film. By forming the charge transfer electrodes of two layers of conductive films this way, the step heights due to the charge transfer electrodes can be reduced as compared with a conventional equivalent where the charge transfer electrodes were made of three layers of conductive films.
Because the first conductive film is processed so as to have a strip pattern and the second conductive film is formed after the processed first conductive film is thermally oxidized, the first-conductive film can be isolated from the second conductive film by the thermal oxidation film of the first conductive film.
Because the first conductive film is isolated from the second conductive film by the thermal oxidation film of the first conductive film and the second conductive film is processed so as to overlap with the first conductive film, the distance between the charge transfer electrodes on the channel region is determined by the thickness of the thermal oxidation film, which enables the distance to always remain constant. Moreover, the distance can be made smaller than the minimum dimension the lithography process will allow.
The length of each of the charge transfer electrodes on the channel region is influenced only by the lithography process in processing the first conductive film, but not by the other lithography processes. This prevents the length of each charge transfer electrode from varying with the mating accuracy of the lithography processes. As a result, the characteristic of transferring charges can be stabilized.
After the first to fourth charge transfer electrodes are formed by processing the first and second conductive films into a strip pattern, the second conductive film, thermal oxidation film, and first conductive film are removed using as a mask a resist film having an opening on the light-receiving element region, which eliminates the necessity for the mating margins for patterning, unlike a conventional manufacturing method where each conductive film on the light-receiving element region was removed by separate patterning. This reduces the area of cells, helping producing a highly integrated solid-state imaging device. When the cell area is not reduced, the area of the light-receiving element can be increased, improving the sensitivity of the solid-state imaging device.
With a conventional manufacturing method, the first to fourth charge transfer electrodes were formed of three layers of conductive films. Therefore, it was difficult to make an opening on the light-receiving element region by etching the three layers of conductive films and the insulating films between the layers using the same resist film as a mask after processing the three layers of conductive films into a strip pattern, because the resistance of the resist film might not be sufficient. In contrast, with the manufacturing method of the present invention, because two layers of conductive films are used to form the first to fourth charge transfer electrodes, an opening can be made easily on the light-receiving region by etching the first and second conductive films and the insulating film between them using the same resist as a mask.
Furthermore, with the method of manufacturing solid-state imaging devices according to the present invention, in the step of removing the second conductive film to form the first to fourth strip charge transfer electrodes, since anisotropic etching is performed using as a mask the first resist film having an opening on part of the insulating film parallel to and partly overlapping with the first strip conductive film and remaining on the second charge transfer electrode and on the region between the fourth charge transfer electrode and the first charge transfer electrode in a set of charge transfer electrodes adjacent to the fourth charge transfer electrode, the second conductive film remains on the sidewalls of the fourth charge transfer electrode and first charge transfer electrode in the region between the fourth charge transfer electrode and the first charge transfer electrode adjacent to the fourth charge transfer electrode, thereby enabling a conductive material portion to be formed of the remaining second conductive film. Therefore, the conductive material portion makes the distance between the fourth charge transfer electrode and the first charge transfer electrode smaller than the minimum dimension the lithography process will allow. As a result, the transfer efficiency of charges can be improved.
Additional objects advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.